1. Field of Invention
The present invention relates to an apparatus for determining and/or monitoring foreign structures in a fluid or in a fluid flow as well as in particular to a method herefor.
2. Description of the Prior Art
It is of importance in an extracorporeal blood circuit such as a hemodialysis system that blood is monitored with respect to dangerous components which can arise by the treatment. Strict care must in particular be taken that gas bubbles and blood clots in the extracorporeal blood circuit can be recognized and/or held back. Known treatment systems are in this respect in a position to react to the recognition of air bubbles and to effect a corresponding alarm and/or treatment stop. It is furthermore attempted to hold back blood clots by means of clot catchers.
Prevalent examinations of blood samples with respect to their components or their flow characteristic can e.g. take place using ultrasound measurements based on different principles or by optical measurements. Gas bubbles in the blood can e.g. be recognized by correspondingly adjusted frequencies using an ultrasound analysis. The occurrence of blood clots can, in contrast, not be detected using the same ultrasound principle since the scattering of the sound at the clots is too small as a rule. It can also be the case that the density difference in the blood section of clots cannot be distinguished from other sections. Optical measurements can recognize the occurrence of clots by a change in the transmission of light. Clots cannot, however, simultaneously be distinguished from gas bubbles which likewise cause an increase in transmission.
It is problematic that, despite anticoagulation, it is not precluded that blood clots occur in the extracorporeal blood circuit of a dialysis machine. The reasons for this can be varied and the incorrect dosage of anticoagulants such as heparin or citrate is not always the cause. Hemostasis, stagnation zones and the contact with air or with the artificial surfaces of the extracorporeal blood circuit promote an activation of the coagulation cascade.
The thrombocytes or blood platelets in the blood change their shape after the activation of coagulation and aggregate to thrombi. These thrombi can clog the capillaries of the dialyzer and, if they are not caught in the extracorporeal blood circuit, enter into the patient's circulation and close smaller vessels there. For this reason, there are venous clot catchers in many current blood tube and cassette systems which should catch blood clots before they enter into the body of the patient. This protective measure is, however, not without controversy.
If it is intended to return the blood not through the vein, but through the artery after a dialysis treatment, a mechanical clot catcher at the arterial side does not provide any remedy since clots previously trapped are released again on the flow reversal and can enter into the patient.
Some approaches are already known from the prior art for monitoring a fluid flow for gas bubbles or solid bodies which arise therein.
U.S. Pat. No. 4,122,713, for example, discloses a measurement system for determining the flow speed of a fluid. In this respect, the system is also able to determine the presence of air bubbles in the blood in addition to the measurement of the blood flow speed.
EP 0 979 111 B1 relates to an apparatus for the optical recognition and quantification of microbubbles. In this respect, an optical recognition of blood clots is described which can, however, in this form only be accepted by particles of a corresponding size, but not for small blood clots.
JP 7103967 describes an arrangement for examining blood serum samples which are located in a sample vessel such as e.g. a measuring cup. A level measurement and a phase boundary detection of separated blood samples located in the sample vessels is carried out by means of optical sensors and an ultrasound sensor.
A system is furthermore known from WO 2007/121398 A2 for detecting gas bubbles and solid particles in blood by radio wave analysis.
U.S. Pat. No. 3,935,876 discloses an apparatus for monitoring a blood flow with respect to air bubbles using optical means, wherein a monitoring for blood clots can take place simultaneously using the apparatus.
Despite the previous approaches, it would be desirable also to recognize the occurrence of blood clots, in particular also small blood clots, as well as gas bubbles in blood fast and reliably, wherein a corresponding measurement device has as simple a structure as possible.